The present invention relates to hydrostatic piston machines used in power transmission and transaxle product and is concerned with an improved pump and motor configuration Although in most instances, single motor hydrostatic transmissions are the normal, some applications do exist where two hydraulic motors are required.
Hydrostatic transmissions act as means for converting rotary mechanical motion, typically provided by an internal combustion engine, to fluid motion by means of a shaft driven hydraulic pump and where the pump is fluidly connected to one or more hydraulic motors. The volume and direction of the fluid delivery to the motor or motors being governed by the displacement setting of the pump, and the motor is the mechanism for converting the fluid motion back to mechanical rotary motion. By regulating both the amount and direction of fluid motion between the pump and motor, usually achieved through the use of a variable-displacement pump, the output speed and rotational direction of the motor can be precisely controlled at a set input rotational speed of the pump. This device is useful in driving vehicles such as lawn and garden tractors or any apparatus requiring precise speed control. In many such applications, the hydrostatic transmission is ideally integrated as part of an overall transaxle driving apparatus which further incorporates speed reducing means and, in the case of the single motor arrangement, a differentialled axle as shown in U.S. Pat. No. 4,979,583 entitled Variable Speed Transaxle.
Hydrostatic transmissions of the type which are fully integrated in a transaxle housing structure containing speed reduction gearing and mechanical differential are fast becoming the preferred driving apparatus for lawn tractors and the like, and are currently either of the type where the hydrostatic transmission utilises an axial piston swash-plate configuration for both the pump and motor or a radial piston configuration for both the pump and motor.
In the axial piston variety of integrated hydrostatic transaxles, the pump and motor are fluidly connected together by means of a center-section which generally is a "L" section configuration containing both fluid passages and valves as well as valve-plate surfaces either formed integral to it on two exterior surface arranged perpendicular to one another on the center-section or as separate components mounted and fixed to such surfaces on the center-section. An example of such axial piston hydrostatic transaxles is shown in the Okada et al. U.S. Pat. No. 4,914,907 and units built to this design, although proven to be both durable and performance worthy, are still significantly more expensive to produce compared to the less sophisticated mechanical gear-shift transmission/transaxle.
At the present time, the need to include such a center-section, which due to its complexity, is the single most expensive item required in the axial piston pump and motor configuration, is a serious obstacle hindering further price reduction. A further problem exists because center-sections are constructed from die-cast or cast aluminium material and porosity in the material surrounding the high-pressure internal fluid passages can, if present to any significant degree, result in a loss of operating efficiency due to the leakage of high-pressure fluid. A still further problem presents itself when the aluminium material is used as the operating surfaces on which the cylinder-barrels revolve. As aluminium is not so durable as compared to steel or iron, the surfaces can wear out prematurely, again resulting in a loss of operating efficiency due to increased leakage of high-pressure fluid.
With respect to the alternative type of hydrostatic unit currently sold in the market and which employs a radial piston pump and radial piston motor configuration, the fluid connection is, by contrast, achieved simply and more economically by using a pintle-valve. Such a pintle-valve is simple to produce and compares extremely favourably in terms of manufacturing cost with their center-section counterpart used in the axial piston design. Pintle-valves are constructed in steel and are heat treatable to provide a very strong and hard wearing operating surfaces against which the cylinder-barrels revolve. Even so, because the standard orientation in the drive-line installation for almost all lawn and garden vehicles, irrespective whether the hydrostatic transaxle uses radial pistons or axial pistons, requires a 90 degree shift in the rotational axis between the input shaft and the axle output shafts, the radial piston type of hydrostatic unit requires additional components in the form of bevel gearing. Such bevel gearing can be positioned behind the hydraulic radial piston motor as is shown in Thoma et al. U.S. Pat. No. 4,979,583 or alternatively in front of the hydraulic radial piston pump as is shown in von Kaler et al. U.S. Pat. No. 5,078,659 However, in either arrangement, the need to include such bevel gearing increases the selling price of the product and may unless the gearing correctly mesh, result in higher than desirable generated gear noise during operation.
The present invention is directed at overcoming the above described disadvantages by way of using the best features of the radial piston pump configuration and the best features of axial piston motor configuration whereby the center-section of the type shown in Okada et al is redundant and the bevel gears shown in Thoma et al or von Kaler et al are redundant. A desirable feature of the radial piston pump is that high speed rotation of the piston carrying cylinder-barrel produces a centrifugal effect on the pistons propelling them radially outwards from their cylinders to improve the draw of fluid into the cylinder. A desirable feature of the axial piston motor design having stick or cylindrical pistons is that the stroke to piston diameter ratio is always greater than the radial piston motor. As a result, based on the same diameter of piston and same number of cylinders, the axial piston motor provides an automatic hydraulic displacement reduction whenever fluidly coupled to a radial piston pump, and this has the great advantage that less speed reduction is needed between the motor and the output power transmission shaft or shafts.